Trends in modem integrated circuit (IC) technology demand increasingly dense ICs, such as for computer systems, portable electronics, and telecommunications products. IC fabrication includes, among other things, photolithography for selective patterning and etching of photoresist layers. The patterned photoresist layer serves as a masking layer such that a subsequent IC processing step is carried out on only those portions of the underlying IC that are uncovered by photoresist, as described below.
A photoresist layer is typically formed on an underlying integrated circuit substrate. The photoresist layer overlays any structures that are already formed on the substrate. Portions of the photoresist are selectively exposed to light through a lithographic mask that includes clear and opaque portions forming a desired pattern. Light is transmitted through the clear portions of the mask, but not through the opaque portions. The incident light changes the chemical structure of the exposed portions of photoresist. A chemical etchant, which is sensitive to only one of the exposed and unexposed portions of the photoresist, is applied to the photoresist to selectively remove those portions of the photoresist to which the chemical etchant is sensitive. As a result, portions of the photoresist which are insensitive to the chemical etchant remain on the IC. The remaining portions of the photoresist protect corresponding underlying portions of the IC from a subsequent IC processing step. After this IC processing step, the remaining portions of the photoresist layer are typically removed from the IC.
High density ICs require sharply defmed photoresist patterns, because these patterns are used to define the locations (and density) of structures formed on the IC. However, light reflects from the surface of the underlying substrate on which the photoresist is formed. Certain structures that are formed on the underlying substrate are highly reflective such as, for example, aluminum layers for forming circuit interconnections. Reflections from the surface of the substrate underlying the photoresist causes deleterious effects that limit the resolution of photolithographic photoresist patterning, as described below.
First, reflections cause the light to pass through the photoresist at least twice, rather than only once. In other words, light first passes through the photoresist to reach the surface of the underlying substrate. Then, light is reflected from the surface of the underlying substrate and passes back through the photoresist layer a second time. The chemical structure of the photoresist changes differently when light passes through the photoresist more than once, rather than when light passes through the photoresist only once. A portion of the light, already reflected from the surface of the underlying substrate, can also reflect again from the surface of the photoresist, passing back through the photoresist yet again. In fact, standing light waves can result in the photoresist from superpositioning of incident and reflected light rays. This overexposure problem is sometimes referred to as the "swing effect."
Even more problematic, the reflections of the light are not necessarily perpendicular. Light reflects angularly from the surface of the underlying substrate, even if the light is incident exactly perpendicular to the surface of the substrate. This results from the diffractive nature of light (i.e., light bends). Off-angle reflections reduce the sharpness of the resulting photoresist pattern. A portion of the light reflected obliquely from the surface of the underlying substrate can also be again reflected obliquely from the surface of the photoresist. As a result of such angular reflections, the light can travel well outside those photoresist regions underlying the transmissive portions of the photolithographic mask. This potentially causes photoresist exposure well outside those photoresist regions underlying transmissive portions of the photolithographic mask. This problem, which is sometimes referred to as "notching," results in a less sharply defmed photoresist pattern that limits the density of structures formed on the integrated circuit. There is a need to overcome these photolithographic limitations to obtain the benefits of high resolution photolithography and high density integrated circuits.